Codegen: Copy and restore the ValueMap and ClastVars explicitly

When generating OpenMP or GPGPU code the original ValueMap and ClastVars must be
kept. We already recovered the original ClastVars by reverting the changes, but
we did not keep the content of the ValueMap. This patch keeps now an explicit
copy of both maps and restores them after generating OpenMP or GPGPU code.

This is an adapted version of a patch contributed by:
Armin Groesslinger  <armin.groesslinger@uni-passau.de>

llvm-svn: 167213